1. Field of the Invention
This invention relates in general to phase-locked loop circuits, and more particularly to a phase-locked loop circuit which exhibits a wide capture range and a low quality factor (Q) to prevent ringing and improve stability.
2. Description of Related Art
Digital data transmission has become increasingly important in the modern communications era. All digital communication systems require some degree of synchronization to incoming signals by receivers. At the heart of all phase synchronization circuits is some version of a phase-locked loop (PLL).
FIG. 1 illustrates a schematic of a basic phase-locked loop circuit 100. Phase-locked loops are servo-control loops, whose controlled parameter is the phase of a locally generated replica of the incoming carrier signal. Phase-locked loops have three basic components: a phase detector 102, a loop filter 104 and a signal-controlled oscillator 106. The phase detector 102 is a device that produces a measure of the difference in phase between an incoming signal 110 and the local replica 120. As the incoming signal 110 and the local replica 120 change with respect to each other, the phase difference 130 (or phase error) becomes a time-varying signal into the loop filter 104. The loop filter 104 governs the response of the phase-locked loop 100 to these variations in the error signal 130. A well-designed phase-locked loop 100 should be able to track changes in the phase of the incoming signal 110, but not be overly responsive to receiver noise. The signal-controlled oscillator 106 is the device that produces the carrier replica 120. The signal-controlled oscillator 106 is an oscillator whose frequency is controlled by a voltage or current level 140 at the input of the signal-controlled oscillator 106.
In many phase-locked loops, the oscillator is driven using a current signal. However, in an IC implementation the center frequency of oscillation varies widely with IC processing. This requires a wide capture range for the phase-locked loop 100, which implies that the control current supplied by the loop filter 104 to the current-controlled oscillator 106 can be high. When the control current of the loop filter 104 is high, the transconductance of transistors in the loop filter that generate the current are also high. In traditional implementations, creating the wide capture range can cause a Q factor which is too high. This high Q causes the phase-locked loop to either ring excessively or become unstable.
A traditional solution for this problem is to use two phase-locked loops whose current- (or voltage-) controlled oscillators are closely matched. However, this increases the area, power, and noise from the extra phase-locked loop.
Thus, there is a need for a phase-locked loop exhibiting a wide capture range and low Q without adding surface area needed to fabricate the phase-locked loop, dramatically increasing the power consumption or increasing noise to the overall phase-locked loop circuit.